Gas turbines can include one or more combustion chambers, wherein a fuel is injected, mixed to an air flow and combusted to generate high pressure flue gases that can be expanded in a turbine.
During operation pressure oscillations can be generated that may cause mechanical damage to the combustion chamber and limit the operating regime.
For at least this reason, combustion chambers can be equipped with damping devices, such as quarter wave tubes, Helmholtz dampers and acoustic screens, to damp these pressure oscillations.
With reference to FIG. 1, a known Helmholtz damper 1 can include an enclosure 2, that defines a resonator volume, and a neck 3 to be connected to a combustion chamber. Combustion and pressure oscillations can occur in the combustion chamber and can require damping. Reference 4 indicates the wall of the combustion chamber.
The resonance frequency (i.e., the damped frequency) of the Helmholtz damper can depend on the geometrical features of the resonator volume and neck and correspond to the frequency of the pressure oscillations generated in the combustion chamber.
The frequency of the pressure oscillations can slightly change from gas turbine to gas turbine and, in addition, also for the same gas turbine it can change during gas turbine operation. For example, at part load, base load, and transition.
At a low frequency range a damping frequency bandwidth of the Helmholtz dampers can be narrow, such that frequency shifting of pressure oscillations generated in a combustion chamber could render a Helmholtz damper connected to it and having a prefixed design resonance frequency, ineffective.
Thus, it can be beneficial to provide a Helmholtz dampers which allows tuning of the resonance frequency.
In order to tune the resonance frequency (to follow the frequency of the pressure oscillations generated in a combustion chamber), Helmholtz dampers have been developed having an adjustable volume.
WO2005/059441 discloses a Helmholtz damper having two cup-shaped tubular bodies mounted in a telescopic way.
EP1158247 discloses a Helmholtz damper whose resonance volume houses a flexible hollow element whose size may be changed by injecting or blowing off a gas. Changing the size of the flexible hollow element can allow the size of the resonance volume to be changed.
U.S. Patent Application Publication No. 2005/0103018 discloses a Helmholtz damper whose resonance volume is divided into a fixed and a variable damping volume. The variable volume may be regulated by an adjustable piston.
These arrangements can be demanding in terms of space for installation and of complex realisation.
Alternatively, tuning of the resonance frequency can be achieved by adjusting the neck of the Helmholtz dampers.
In this respect, EP0724684 discloses a Helmholtz damper in which the cross section of the neck may be adjusted.
EP1624251 discloses a Helmholtz damper with a neck whose length may be adjusted by overlapping a plate including holes to its mouth.
These arrangements are complex and, in addition, they do not allow a fine tuning of the resonance frequency to follow small shifting of the frequency pressure oscillations in the combustion chamber.